The rapid pace of development in wireless communication systems has typically led to significant changes to the standards which define the operation of such systems. For example, the standards defining the operation of IS-95 CDMA wireless systems have progressed from TIA/EIA IS-95A to TIA/EIA IS-95B, and are now in the process of moving toward TIA/EIA IS-2000, also known as IS-95C. The IS-95A, IS-95B and IS-95C standards are collectively referred to herein as IS-95. Other CDMA standards, such as Multi-Carrier (MC) cdma2000 and the next-generation European standard known as Universal Mobile Telecommunication System (UMTS), are also being proposed.
These related standards each generally define an air interface specification that allows a mobile unit to communicate with a base station associated with a cell site. The interface definition typically includes a set of air interface channels, channel signal encoding rules, and signaling messages to enable the mobile unit to place and receive voice or data calls to and from a land line network, as well as to and from other mobile users.
FIG. 1 shows an example of a base station 100 configured in accordance with the above-noted IS-95 standard. The base station 100 includes a control computer 102, a control and traffic bus 104, and a set of M channel unit boards 106-i, i=1, 2, . . . M. The control computer 102 interfaces with a mobile switching center (MSC) which provides a link to other base stations and to a public switched telephone network (PSTN). In an IS-95 CDMA system, spread spectrum digital signals from different user calls on a given base station antenna sector are added together to generate a composite spread spectrum digital signal for that sector. The composite spread spectrum digital signal is generated by one or more of the channel unit boards 106. The base station design of FIG. 1 allows the channel unit boards 106 to communicate signals from one such board to the next in support of users on one CDMA carrier, designated C1, and up to three 120° antenna sectors, designated α, β and γ. Three sector systems are commonly used in practice, although omni-directional and two-sector systems may also be deployed. The use of a larger number of sectors, such as six sectors, is less common, but also possible.
Within each channel unit board 106-i in the base station 100 of FIG. 1, the spread spectrum digital signals of up to N users are added together on a per-sector basis. For each sector, the summed spread spectrum digital signals of users served by a particular channel unit board 106-i are added to the respective signals from the previous channel unit board, i.e., the channel unit board to its left in the FIG. 1 design. The summed digital signals are output from the channel unit board 106-i, and become inputs to the next-in-line channel unit board 106-(i+1) closer to a set of three radio boards 108-1, 108-2 and 108-3 in FIG. 1. Therefore, up to N users per channel unit board are added together by the mechanism of summing the signals from channel unit board to channel unit board. In a design with M such channel unit boards, each supporting up to N users, up to M×N total users can be supported on the three sectors α, β and γ. The interconnections between the channel unit boards are provided by a transmit digital signal communications bus denoted Tx-bus.
It should be noted that although the description of the base station 100 of FIG. 1 is directed primarily to its transmit operations, similar interconnection issues arise with respect to receive operations. The corresponding receive bus (Rx-bus) is omitted from FIG. 1 for clarity and simplicity of illustration.
The digital processing elements on each of the channel unit boards 106-i can be used to support a user call on any of the three sectors α, β and γ. This capability is referred to herein as channel element pooling, or simply channel pooling, and in the FIG. 1 design, is applied to one carrier and three sectors. Such an arrangement is more particularly referred to as single-carrier/multi-sector channel pooling.
In FIG. 1, digital in-phase (I) and quadrature phase (Q) signals for each of the three sectors α, β and γ and the one CDMA carrier C1 are summed from channel unit board to channel unit board, and finally are passed to one of the three radio boards 108-1, 108-2 and 108-3, depending on the sector. Each radio board 108-1, 108-2 and 108-3 converts the digital I and Q signal inputs into an RF signal. The RF signals for sectors α, β and γ are then amplified by power amplifiers 110-1, 110-2 and 110-3, filtered in transmit filters 112-1, 112-2 and 112-3, and radiated by transmit antennas 114-1, 114-2 and 114-3, respectively. Other types of conventional techniques may be used to communicate signals among the channel unit boards, e.g., the I and Q signals for each sector may be multiplexed onto one back plane trace.
Conventional IS-95 CDMA base stations such as base station 100 of FIG. 1 typically support channel element pooling only at the single carrier level, i.e., single-carrier/multi-sector channel pooling. FIG. 2 shows the transmit direction interconnection between channel elements, more specifically referred to herein as cell site modems (CSMs), of a given channel unit board 106-i of base station 100. Each of the N channel elements of a given channel unit board 106-i generally supports a single voice or data call for a particular one of N users, and may correspond to, e.g., a single integrated circuit or a portion of an integrated circuit. In FIG. 2, channel elements 120-N, 120-(N−1) and 120-(N−2) are interconnected in a “daisy chain” arrangement as shown. Although not shown in FIG. 2, one or more additional chains may be provided for redundancy in case an element of a given chain fails. A given channel element of the exemplary chain shown in FIG. 2 combines its own outputs for the three sectors α, β and γ with the corresponding outputs of the previous element of the chain. The resulting combined outputs are then delivered to the next element in the chain. The output of the last element in the chain, i.e., element 120-N in this example, is delivered to the system backplane or to a suitable board combiner for further processing as previously described.
FIG. 3 shows an example of the interconnection of channel elements in the transmit and receive directions. In the receive direction, all the CSMs in a given chain receive the same information, i.e., the CSMs are connected using a broadcast bus rather than a daisy chain. The FIG. 3 arrangement includes two chains A and B of N=10 channel elements each, with the channel elements in each chain denoted CSM0 through CSM9. Each of the channel elements in each of the chains receives baseband receive data for each of the three sectors α, β and γ. Since the transmit and receive channels are symmetric, the same carrier assigned for the transmit direction is also assigned for the receive direction.
A significant problem with the conventional single-carrier/multi-sector channel pooling arrangements described in conjunction with FIGS. 1, 2 and 3 above is that when any one of the channel elements in a given chain is assigned to a particular carrier, all the channel elements in that chain, or in this case the corresponding channel unit board, cannot be assigned to any other carriers in the system. Another problem with the conventional arrangements is that a failure of a single channel element in the chain can cause the entire chain to fail. A need therefore exists for a multi-carrier/multi-sector channel pooling arrangement that provides increased system flexibility and reliability relative to conventional single-carrier/multi-sector channel pooling.